1. Field Of the Invention
The present invention relates to a hydrogen absorbing alloy which can absorb and release hydrogen isotopes so as to separate them effectively, and it also relates to a method for the separation of hydrogen isotopes using the hydrogen absorbing alloy.
2. Description of the Prior Art
There are three isotopes of hydrogen: light hydrogen ( symbolized: H), deuterium ( symbolized: .sup.2 H or D) and tritium (symbolized: .sup.3 H or T). Among these hydrogen isotopes, deuterium and tritium are known as heavy hydrogens. The heavy hydrogens and compounds containing them are used for various purposes. For example, they are used as fuels for nuclear fusion, treating media for heavy-water reactors, neutron moderators, baryon beam sources for accelerators, tracers for analysis, and the like. Since light hydrogen and heavy hydrogens are usually present as a mixture, many methods have been developed for separating a heavy hydrogen from a mixture of light and heavy hydrogens. Examples of these methods include: electrolysis of a mixture of water and heavy water, in which the separation of hydrogen isotopes is effected by utilizing the difference in voltage for electrolysis between water and heavy water; a temperature exchange reaction in which a hydrogen exchange reaction is effected between heavy hydrogen sulfide and hydrogen sulfide; hydrogen-water chemical isotope exchange using a hydrophobic platinum catalyst; laser isotope separation which utilizes the difference in decomposition rates between water and heavy water when they are exposed to infrared laser light; distillation of water; distillation of liquid hydrogen; a hydrogen-water isotope exchange reaction, etc. The use of a metal or alloy capable of absorbing hydrogen, i.e., a single metal such as vanadium or a binary alloy such as TiCr.sub.2 and TiNi, has also been proposed for the separation of hydrogen isotopes. In the proposed method, the metal or alloy is first allowed to absorb a mixed gas of hydrogen isotopes (e.g., H.sub.2 and D.sub.2) at a predetermined high pressure (e.g., 10 atmospheric pressure). Then, the pressure is lowered (e.g., 2 to 3 atmospheric pressure) so as to cause the metal or alloy to release only a particular hydrogen isotope of H.sub.2, by utilizing the differences in equilibrium desorption pressures between the respective isotopes. In another proposed method, the concentration of a particular hydrogen isotope (e.g. D.sub.2) in a mixed gas of hydrogen isotopes (e.g., H.sub.2 and D.sub.2) is Increased through the absorption of the mixed gas by the metal or alloy, by utilizing the differences in absorption rates between the respective hydrogen isotopes (Y. Osumi, Hydrogen Absorbing Alloy; Yonoshobo Ltd., Tokyo, Japan).
Among the above-described conventional methods, the hydrogen-water isotope exchange reaction is widely used. This exchange reaction is expressed by the following reaction formula II: ##STR1## In the reaction formula II, the reaction proceeds toward the right at low temperatures, and proceeds toward the left at high temperatures. For example, when a gas containing tritium is allowed to stand in contact with water at a low temperature, tritium can be collected in the form of water of the formula HTO. The dual temperature exchange method and the single temperature exchange method are known as isotope separation processes using this principle.
The conventional methods described above, however, have disadvantages. For example, the temperature exchange method using hydrogen sulfide requires complicated operations for temperature control and usually requires a high temperature of about 200.degree. C. The use of a poisonous substance such as H.sub.2 S is another disadvantage of the method. The method also requires large-scale equipment and high cost, and furthermore, Involves problems of environmental pollution due to the production of hydrogen sulfide and ammonia. In the above conventional method using the metal or alloy capable of absorbing hydrogen, it is extremely difficult to attain a high concentration of a desired hydrogen isotope because of the small differences in equilibrium desorption pressures between the respective hydrogen isotopes. Thus, the same separation process is required to be repeated over and over again. Such repetition of the same process causes the deterioration of the metal or alloy.